I propose to study the physiological mechanism of stimulus-response coupling in contractile cells of vertebrate and invertebrate animal, with emphasis on the relevance of this mechanism to muscle function in general. The objective to be achieved and methods to be used are as follows: Measure certain physical and chemical features indicative of contractile activation at the subcellular level, and correlate these things with changes in transmembrane potential and force development or shortening. The relevant chemical features of activation are the time course and spatial localization of changes in intracellular calcium concentration. These will be determined by introducing the calcium-sensitive bioluminescent protein aequorin into cells and recording the light output by microscopy and photometry.